Monica S. Lewinsky's immunity agreement with independent counsel Kenneth W. Starr provides her with the legal equivalent of a huge umbrella, shielding Lewinsky from any possible prosecution involving the 25-year-old former intern's relationship with President Clinton.

The only exception would be if, when Lewinsky tells her story before the grand jury, prosecutors were to conclude that she is lying. Under those circumstances, even the broad "transactional" immunity that Lewinsky received would not prevent a perjury prosecution.

The deal -- her mother, Marcia K. Lewis, received the same grant -- appears to be just what Lewinsky had asked for all along: full immunity, rather than a more limited form of immunity, known as "use immunity."

Under "use immunity," prosecutors would have been able to force Lewinsky to testify but would not have been allowed to use her statements -- or any evidence or leads they derived as a result of it -- to prosecute her for any underlying offenses she may have committed, such as perjury or obstruction of justice. While that is the more common form of immunity, prosecutors do sometimes provide fuller, "transactional" immunity, particularly when they do not consider the witness involved to have engaged in particularly reprehensible behavior.

As a practical matter, particularly under the strict interpretation of the use immunity laws that prevails in the federal courts in the District of Columbia, Starr's office might have encountered obstacles in actually prosecuting Lewinsky if he had given her use immunity.

Still, said George Washington University law professor Stephen Saltzburg, "It's pretty clear, given the tapes, the talking points and Linda Tripp, they could have given use immunity and convicted Monica and potentially her mother of witness-tampering, even if they didn't use anything she said to the grand jury against her. They're prepared to forgo what likely would have been an opportunity to convict. That suggests they must feel that they're getting complete cooperation."